Test and sensitivity analysis of base-isolated steel frame with low-friction spherical sliding bearings

Abstract

In Japan, spherical sliding bearings with low friction coefficients are gaining popularity for base-isolating low-rise frames that are relatively lightweight. However, the complete dataset of isolators and a base-isolated frame for evaluating the model sensitivity and response uncertainty are limited. This study first presents the design process, isolation unit- and frame-level testing, and a blind prediction contest conducted on the occasion of full-scale shaking table testing of a three-story base-isolated hospital specimen. The design process utilizes a numerical model that accounts for the velocity and contact pressure dependencies and requires soft- and hard-case simulations with nominal friction coefficients plus and minus standard deviation to consider the uncertainties associated with the bearing behavior. The pre-shipment isolation unit and frame shake table testing yielded an invaluable dataset for bearings under normal and low contact pressures, low and high velocities, and constant and varying axial loads. The accompanying blind prediction contest provided a valuable dataset for rethinking the impact of modeling uncertainty. In-depth data analysis and sensitivity analysis were conducted. The sliding coefficient increased under low-contact pressure and low-velocity conditions. The static friction coefficient was 1.9 to 4.5 times higher than the dynamic coefficient, but this had little impact on the residual displacement, cumulative travel, and maximum story shear force. The axial force fluctuation, vertical motion, and two-directional input did not significantly affect the bearing behavior in the test. The test and the following simulations confirmed that the low friction coefficient helped the building contents, that is, medical equipment in this study, remain in order under near-fault and long-period ground motions.